Short backfire antenna



April 21, 1970 H. w. EHRENSPECK I SHORT BACKF IRE ANTENNA 3 SheetsShe-et 1 iled Feb. 28, 1968 A ril 21, 1970 H w. EHRENSPECK 3,

SHORT BACKFIRE ANTENNA "'iled Feb. 28, 1968 v 3 Sheets-Sheet a 1 N VE N TOR. IVER/W174 MM 4 26 m? fl m 3,508,278 SHORT BACKFIRE ANTENNA Hermann W. Ehrenspeck, 94 Farnham St., Belmont, Mass. 02178 Filed Feb. 28, 1968, Ser. No. 708,910 Int. Cl. I-Ilq 19/30, 19/10 US. Cl. 343-819 14 Claims ABSTRACT OF THE DISCLOSURE A directional antenna system in the form of a combination of a cavity-type antenna and a slow-wave endfire antenna to provide a slow-wave structure energized by an extremely efiicient feed in the form of a cavity radiator.

BACKGROUND OF THE INVENTION This invention relates generally to directional antennas and more particularly to a combination of cavity-type antenna serving as a feed and a slow-wave endfire antenna energized thereby.

In the prior art, there exist slow-wave endfire antennas, for example, a Yagi, a helix or double helix, a dielectric rod, or a disk-on-rod structure. Simultaneously there also exists a cavity antenna functionally and structurally similar to that of the reflection antennas described in pending US. patent application No. 446,128 filed Apr. 6, 1965, and now abandoned and also described in US. Patent Nos. 3,122,745 and 3,218,646 issued Feb. 25. 1964 and Nov. 16, 1965, respectively. However, there does does not exist a combination of the two aforementioned antennas. In addition thereto, the prior cavity-type antennas had limitations including bandwidth, and compact- SUMMARY OF THE INVENTION In accordance with the present invention there is provided a directional antenna system which is obtained by combining two antenna types. One of them is a slow-wave endfire structure, the other is a cavity-type antenna whose configuration resembles a strongly modified Fabry-Perot cavity resonator similar to that used as part of a Laser. The function of the resulting new antenna type, referred to as cavity-endfire antenna, is best described and analyzed by stating that its slow-wave endfire structure is energized by an extremely efficient feed in the form of the cavity radiator which alone has a higher gain than a five wavelengths long endfire antenna. Particular advantages of the cavity-endfire antenna are its low side and back lobes and its high gain which is obtained with minimal length and cross-section of the antenna structure.

In addition to the foregoing, the present invention also provides an improved cavity-type antenna with increased bandwidth and compactness. The addition of a tuning plate thereto increases the frequency bandwidth substantially over prior models permitting the utilization thereof in countermeasure operations. The combination of the Yagis and backfire design to make a smaller flat plate of the cavity-type antenna reduces the overall diameter of the structure Where this kind of compactness can be traded for length. Finally, flush mounting techniques are described which are of very practical worth for supersonic aircraft and missiles.

An object of the invention is to provide a directional antenna system including the combination of a'cavity- Patented Apr. 21, 1970 Yet another object of the present invention is to provide a directional antenna wherein a slow-wave structure is energized by an extremely eflicient feed in the form of a cavity-type antenna.

The various features of novelty which characterize this invention are pointed out with particularity in the claims annexed to and forming part of this specification. For a better understanding of the invention, however, its advantages and specific objects obtained with its use, reference should be had to the accompanying drawings and descriptive matter in which is illustrated and described preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 shows a modified version of a cavity resonator utilized to describe the cavity-type antenna of this invention;

FIGURE 2 illustrates one extremely compact cavitytype antenna;

FIGURE 3 illustrates the measured E- and H-plan radiation patterns provided by the cavity-type antenna of FIGURE 2;

FIGURE 4 shows a second cavity-type antenna providing amongst other features a wider bandwidth;

FIGURE 5 is a third cavity-type antenna which can also be mounted flush into a body;

FIGURE 6 shows a fourth cavity-type antenna which extends only about .25 wavelength into a body;

FIGURE 6a shows a front View of the cavity-type antenna of FIGURE 6;

FIGURE 7a shows a slow-Wave endfire antenna in the form of a Yagi which is utilized in combination in the present invention;

FIGURE 7b illustrates one cavity-type of antenna to be combined with the Yagi of FIGURE 70:;

FIGURE 70 shows one preferred embodiment of the cavity-endfire antenna of the present invention;

FIGURE 8 shows a second preferred embodiment of the cavity-endfire antenna;

FIGURE 9 shows a third preferred embodiment of the cavity-endfire antenna with the two slow-wave structures, one above the other;

FIGURE 10a shows a front view of one of the arrangements of the combination cavity and slow-wave endfire antennas having two slow-wave structures, one above the other;

FIGURE 10b shows a front view of another one of the arrangements of the combination cavity and slow-wave endfire antennas having two slow-wave structures, side by side.

FIGURE 100 shows a front view of still another one of the arrangements of the combination cavity and slowwave endfire antennas having two slow-wave structures, one above the other, and two side by side;

FIGURE 10d shows a front view of yet another one of the arrangements of the combination cavity and slowwave endfire antennas having four slow-wave structures DESCRIPTION OF THE PREFERRED EMBODIMENT Now to describe the theory of operation of the cavitytype antenna, the Fabry-Perot cavity resonator which is mainly used in optical interferometry, is a parallel-plate structure consisting of large perfectly reflecting walls. A

modified version of this resonator has lately found wide application as a laser resonance cavity. A typical example is schematically shown in FIGURE 1. M is a perfectly reflecting plate and M a partially reflecting plate, both mounted exactly transversely to the longitudinal axis (g) of the structure and spaced many wavelengths apart. The space between M and M constitutes the cavity which for laser applications has to be filled with the active laser material A. The cavity length L has to be adjusted so that multiple reflection of the laser energy occurs in the space between M and M A portion of the cavity energy is radiated from the side of the partially reflecting plate M with a pattern of extremely high directivity in the longi tudinal axis (g) of the structure.

It has been found that a cavity configuration which is, in fact, a strongly modified version of that of FIGURE 1 canwithout the active mediumbe used as a very efficient cavity antenna and as such serve as the feed of the cavity-endfire antenna described in this invention. U'nfortunately, the dimensions of FIGURE 1 cannot be simply scaled for any frequency range because they are, for most applications, prohibitively large in terms of wavelength. An extensive investigation of the field distribution inside the cavity has shown, however, that it can tolerate further modifications Without losing its basic characteristics as a resonance cavity: its length may be drastically reduced to as little as one half of a wavelength; the area of plate M which may be a planar reflector may be reduced to a few square wavelengths and the area of M to about one quarter of a square wavelength; alternatively, plate M may be in the form of and replaced by a row of simple rod reflectors spaced up to approximately one half wavelength from each other and arranged in the plane of M As a result the extremely compact cavity antenna is otbained whose configuration is shown in FIGURE 2. It constitutes, in fact, the shortest possible cavity antenna of this type, M marks a circularly shaped planar reflector-corresponding to M in FIGURE 1and M a reflector combination consisting of three rod reflectors R,

S, and T, which are arranged in a plane transverse to the longitudinal axis (h) of the antenna-corresponding to M in FIGURE 1. Reflector M may also be a planar disk of not more than 6 of the area of M The axial length L of the cavity which is terminated by the planes of M and M is about .50 wavelength. F is the cavity feed. Its position, usually at a spacing of .25 wavelength from M is not very critical and may for best input impedance matching of the antenna be moved nearer to or further away from M The performance of this short cavity antenna is essentially improved by a rim B of approximately .25 to .50 wavelength width which surrounds M With a diameter of two wavelengths for M and with optimum adjustment of all parameters the cavity antenna of FIGURE 2 yields a gain of more than db above an isotropic source, The measured E- and H-plane radiation patterns are presented in FIGURE 3. All sidelobes are at least db below the maximum in the E, as well as in the H plane, and the backlobe is more than 30 db below the maximum. The circular shape of the planar reflector M is partly responsible for these exceptionally clean patterns, but squareor polygon-shaped reflectors which are easier to build also give satisfactory results for most practical applications.

The antenna configuration of FIGURE '2 is functionally and structurally similar to that of the reflection antennas described in Patents No. 3,122,745 and 3,218,646. At the time the patent application was submitted, however, it was not yet known that the key to the high-gain performance of this antenna type was the optimization of its cavity characteristics. This knowledge led to some essential improvements in construction and performance which were achieved by further modifications of the cavity configuration. Some of them are of special importance for the use of this cavity antenna as feed for the cavity-endfire antenna described in this invention. As'described in an earlier invention disclosure also, a corner reflector may be used instead of the planar reflector M It has been found that for many applications rim B of reflector M has to extend only in the direction of the polarization of the field. Therefore a squareashaped reflector M of a linearly polarized cavity antenna requires the rim only along those edges which are parallel to the dipole feed, and a circularly-or polygon-shaped reflector M only along two sectors approximately wide. For a cavity antenna for crossed polarization, however, the rim must surround the entire periphery of reflector M The operating frequency range of a cavity antenna according to FIGURE 2 with a fixed cavity length L is limited. If, however, a variation of L is provided, for example by a mechanical adjustment of the spacing between M and M by rotating supporting rod 10 which is threaded to permit movement in or away from M other cavities described hereinafter may be adjusted in the same manner. The cavity antenna can be tuned for opti: mum performance over, that extended frequency range the feed is capable of. Moving M towards M results in optimum adjustment for higher frequencies, moving it away from M for lower frequencies. With the usable bandwidth defined as that frequency range within which the antenna gain is approximately proportional to the area of the larger reflector M and all sideand backlo'bes are at least 10 db below the maximum in the E- and H-plane patterns, the bandwidth of the optimized cavity antenna of FIGURE 2 is about 1.321 for a fixed cavity length L, and the antenna can be tuned within a frequency range of approximately 2:1 by adjusting the cavity length L.

It has been found, however, that the bandwidth of. a cavity antenna with fixed cavity length L can be essentially increased if the reflector dimensions and the cavity length are optimized for different frequencies within the prescribed frequency range. In a practical model a band-. width of about 1.7:1 was obtained by, optimizing the: reflectors M and M for the highest frequency and the cavity length for about of this frequency. v

A still wider bandwidth can be achieved with the antenna configuration of FIGURE 4. M B, F, M .and L have the same meaning as in FIGURE 2. A third plane reflector M in size and shape about the same as M is arranged outside the cavity at a distance G from, and parallel to M Because of the very complicated interaction of-the various antenna parameters the optimal value of G-in general .25 to .50 times Lhas to be determined experimentally. Of course, the cavity feed has to be feasible for the prescribed frequency range of the.

antenna. For a cavity antenna model with circularly shaped reflectors M M and M for example, the following approximate dimensions (given relative tothe Wavelength k of the highest frequency) shield optimum performance over 2:1 bandwidth Diameter of M =2.0O

Diameter of M =.50

Diameter of M =.457\

Cavity length L=.67 Distance between M and M :G=.16)\

'It has been found that the wide-bandwidth performance of the cavity antenna of FIGURE 4 can be analyzed as the result of a combined application of three different antenna principles, each of them being dominant'in a part of the 2:1 frequency range. With f as the highest frequency, the antenna acts as a cavity antenna to about 0.8 f The cavity effect gradually diminishes with decreasing frequency and, simultaneously, reflectors M and M approach their optimal dimensions as directors at about 0.7f 'In this frequency range the antenna acts as atwodisk array to about 0.673,. For still lower frequencies the disks gradually lose their director characteristics, and the antenna finally acts to below 0.5 f as a simple reflector antenna with the feed as the only effective element in front of reflector M FIGURE 5 is a sketch of aspecial type of cavity antenna whichcan also be mounted flush into a body. M B Ii, F and M have the same meaning as in FIGURE 2.,W'ith B extending,however, over the entire cavity length. P marks a dielectric plate which is parallel to M and connected withthe edge of rim B; it can at the same time support "reflector M The resulting antenna is a completely enclosed structure which can easily be made weatherre'sis'tant. 'As flush-mounted cavity antenna it extends only .50 wavelength into the space of a body. An experimental S ban model according to FIGURE 5 which was covered 'by'a Plexiglas plate with reflector M glued to its surface, showeda small increase in grain over a same sizeca'vity antenna according to FIGURE 2, and a further reduction of its sideand backlobe level.

I FIGURE 6 is a sketch of a cavity antenna which extends only about .25 wavelength into a body. All corresponding elements are marked in the same way as in FIGURE '5'.'The dielectric plate P is located in the plane of Feed F The only element outside the enclosed structu 're' is'reflector M It may be, as shown in FIGURE 6a, attached to the center of the dielectric plate P 'or the reflector M by-a metallic rod Q or may be supported by two 'stjurdyfrods which are connected with rim B. For linear polarization'the'se two' metal rods arranged transverse't o the plane of'polarization-shown in FIGURE 6a asdotted lines -may be used, for crossed polarization, howeverftherods must be made from non-conductive material. v I v To-"obtain the cavity-endfire antenna according to'the invention anyon'e of the described configurations of cavity antennas can be combined with any one of the known types of slow-wave endfire "antennas, for example, with a Yagi, -8] l161lX or double helix, a dielectric end-fire rod structure, or' aldi'sk-on-rod structure. For the following discussions the Yagi was "chosenas an example. It consis'ts', in" 'its simplest form'shown in FIGURE 7a, of a dI'pOlefee'd-F g a single dipole reflector R and a row of directors D, to D forming theslow-wave structure. The

working'p'rinc'iple of this antennatype is sufliciently described-in the literature. Its gain is a function of the length of 'the' slowwave structure provided that the directors are adjusted -t'otheir optimum height, and the feed-reflector combination F Ry isoptimized for maximum directivity iri"the endfire direction. In FIGURE is also shown how thetcavity-endfire antenna of FIGURE 7c is created by nesting- -the-feed end of the Yagi-type endfire'of FIGURE 7z'i into the cavity "antenna 6f FIGURE 7b. In all three fig'tijres'the 'sarneelements are marked by the same letters asfipreviousl'yused. For simplicity in the drawings it is assilmedthat all spacings between reflectors, feeds, and

directors of' both antennas'arethe same; the nesting is thefieffectedin such 'a way that feeds F and F coincide iii-"F," R is replaced by M "and the first of thedireetors, D is replaced by the centerireflector R. What is finally left of theyagi is the slow-wave endfire structure (consisting o'f-thedirectdrsD to D which now extends in fro'nt'of the r'eflector combination R, S, T. The gain of the cavity-endfire antenna now increases as the length of the slow-wave structure is extended, provided that its phase velocity iis progressively. adjusted to the new optirnum required by each particular length. It should be mentioned that .the optimum phase velocity of the cavityendfire antenna is, however, essentially different from that of an equal-lengthconventional endfire antenna. The

specified size. (2) Replacing the first director of the Yagi *reflector which has its rim B extending only along two D by a reflector combination R, S, T or any other realization of M for example a disk. (3) Decreasing the length of the directors of the Yagi to the proper height.

FIGURE 8 is a sketch of a cavity-endfire antenna according to the invention, with a circularly shaped reflector M a combination of three dipoles as reflector M and seven directors, D to D (slow-wave endfire structure). The horizontal direction of the dipoles indicates that this antenna model is feasible for receiving or transmitting horizontal linear polarization. For vertical polaritwo or more slow-wave endfire structures in front of 'the cavity, either one above the other or side-by-side or both ways. FIGURE 9 shows such an antenna configurationwith two slow-wave endfire structures one above the other. All elements are again marked by the same letters as in the previous figures. M is a square-shaped of its edges. F is a broadband bow-tie dipole. It should be mentioned that this arrangement of slow-wave structures in front of the cavity antenna has nothing in common with the well-known arrangement of two or more *Yagis in front of a plane reflector. In such a conventional Yagi array each Yagi antenna is fed by a separate half-wavelength dipole, or every two Yagis by a fullwavelength dipole, and the slow-wave structures have to be arranged directly in front of their dipole feeds.

In contrast, the two or more slow-wave structures of a cavity-endfire antenna are according to the invention energized by the radiation field of the cavity antenna and can be moved nearer to or further away from each other independent of the single cavity feed, thus changing the field distribution in the radiating aperature so as to obtain the desired shape of the radiation patterns.

The arrangement of the two slow-wave structures one above the other (FIGURE 9), or alternatively side by side, narrows the main beam of the radiation pattern in the vertical plane (H plane) or alternatively in the horizontal plane (E plane), leaving it practically unchanged in the other plane. If slow-wave structures are arrayed simultaneously one above the other and side by side, the patterns are narrowed in both planes. Some of the many possible arrangements are shown in FIGURE 10. The circles indicate the circularly shaped reflector M The solid straight lines show the location and direction of dipole slow-wave structures, and the dashed lines the location and direction of the dipole-reflector combination R, S, T in FIGURES l0a-c and of the reflector disks M in FIGURES '10d and e. The cavity feeds which are linear diploes in FIGURES 10a, b, c, and crossed dipoles in FIGURE 10d and e are not shown. While the configurations of FIGURES 10a, b, and c are for linear polarization, those in FIGURES 10d and 2 may be ap plied to any type of polarization the cavity feed is capable of, for example, for linear elliptical or circular polarization.

If two or more slow-wave structures'are used the field distribution'in the radiating aperature of the cavity endfire antenna can be changed by their spacing. Best results are obtained with tapered structures. In general, the increase in gain is proportional to the length and number of slow-wave structures, and the sideand backlobe level of the radiation patterns may be optimized by controlling the phase velocity and spacing of the slow-wave structures. It is noted that the slow-wave structures are labeled D in FIGURES *8, 9, and 10a-10e.

It is emphasized again that in the present invention, the cavity-type antenna that is utilized in combination with the slow-wave endfire structure operates in the resonant cavity concept and its dimensions must be chosen in accordance with the principles of open resonant cavities. To obtain highest gain at a pretermined frequency, the electric length of the cavity must be approximately any multiple, including unity, of half a wavelength. The diameter of the fully reflecting plate of FIGURE 2 must be large enough to set up the desirable resonant modes between the fully and partially reflecting plates of FIGURE 2, and cannot be so small thatrenergy starts leaking out before reaching the virtual aperture, i.e., -the plane through the partially reflecting plate in FIGURE 2. The optimum was found to be approximately 2.2 wavelength diameter for a cavity length of one half of a wavelength. If for the same cavity length the diameter size is increased, the performance of the cavity-.

type antenna deteriorates. If, however, a larger cavity length, for example one wavelength, is chosen, the dia-v closed without departing from the spirit of the invention; as set forth in the appended claims, and that in some.

cases certain features of the invention may be used to advantage without a corersponding use of other features- Having now described my invention what I claim asv new and desire to secure by Letters Patent, is as follows: 1. A cavity-endfire antenna system transmitting and receiving electromagnetic energy in the direction of the longitudinal axis thereof comprising a fullyv reflecting plate, a partially reflecting plate, both of said plates being mounted transversely to said longitudinal axis and spaced from each other to constitute an electromagnetic resonant cavity with the cavity length adjusted so that multiple reflection of said electromagnetic energy occurs in said space, antenna feed means interposed between said plates to energize said cavity, with a portion of the cavity energy being radiated by way of said partially reflecting plate with a radiation field pattern of extremely high directivity along said longitudinal axis, and slow-wave endfire struc ture means being positioned outside of saidcavity adjacent to said partially reflecting plate, said endfire structure means having two ends, one of said ends being disposed in said radiation field pattern for eflicient feeding thereof, and the other of said ends radiating electromagnetic energy in response tosaid feeding.

2. A cavity-endfire antenna system as described in coincides with said longitudinal axis.

3. A cavity-endfire antenna system as described. in claim 1 wherein said slow-, wave endfire structure means is parallel to and'displaced from said longitudinal axis. 4. A cavity-endfire antenna system as described in claim 3 wherein said slow-Wave endfire structure means is a first slow-wave endfire structure means displaced in the vertical direction, and further including a second slowwave endfire structure means identical to said first but displaced in the vertical direction opposite to said first. .5. A cavity-endfire antennasystem as described in claim 3 wherein said slow-wave endfire structure means is a first slow-wave endfire structure means displaced in the horizontal direction, and further including a second slow-wave endfire structure means identical to said first but displaced in the horizontal direction opposite to said claim 1 wherein said SlOWrWflVC endfire structure mean v claim 1 whereinsaid partially'reflecting plate is compris claimil further r d ng a reflecting rim surr'oundiri L partially reflecting .plat'e interposed between. said first 6. Av cavity-endfire antenna. system as desc,'bed ir r. claim 5 further including third and fourth slowje ave'end; fi're' s tructure means also identical to. said firstIsaid third and fourth beingpositioned vertically oneither s said longitudinal axis. e 7. A' cavity-endfire antenna system as, described, in claim 1' wherein said cavity length; approxirnat' o half Wavelength, said' fully reflecting plate is a, circular] mately two wavelengths, and saidpartially reflecting platel being also a circularly shaped planar reflector liavinga:

diameter of approximately a half o fa wavelength. I 8. A cavity-endfire antenna system. as. fd'sc'rib d of a row of rod reflectors spacedfa prede'terminedld tance from each otherand'arranged in the pla'ne'of said par'tially reflecting plate. i

9; A cavity-endfire antenna system as des er e fullyreflecting plate, said rim having a preselected "10. A cavity-endfire antenna system as.described claim 1 wherein said partially reflecting plate is a firs partially reflecting plate, a nd furthervincludinga second tially'reflecting plate and said slow-waveendfire structure means, said second plate also being' parallelto said first; plate and'of substantially the same magnitudetherewith 11. A cavity-endfire antenna system as described n I claim 1 further including means to vary saidcavi gfii, 30 r to provide e," v

12. A cavity-endfire antennajsystem asndes cribedu in claim 15 further includingQa reflecting rim having tvsid" optimum performance over a wide frequency;

.. edges, said rim surroundingsaid fully reflecting platehnd,

connected to one of said edges, and-a dielectric plate;v parallel to said fully reflecting platejconnected t0 -tlle.jOth6 1"- of said edges, said dielectric platetserving as a: support for said partially reflectingplate.v

13. A cavity-endfire antenna system transmittinggand' receiving electromagnetic energy in the-direction of the. longitudinal axis thereof comprising a full y reflecting plate, a first partially reflecting plate, both of said. plates being; mounted transversely tofsaid longitudinal axis and spaced; from each other to constitute an electromagnetic; resonantE cavity with the cavity length. adjusted so-thattmultiple): reflection of said, electromagnetic. energy occurs} in said? space, antenna feed means interposed bep i to energize saidcavity, with a. portion of the .cavity energy; being radiatedby wayof said first partially. reflecting plate,-

with a radiation field pattern of extremely hligh di qi i along said, longitudinal axis, and a? second; partially; reflectaj) ing plate positioned adjacent and. parallel,v to saidfirstv pari tially, reflecting plate at a preselected distance therefrom;v to provide broadband performance. I i I i 14. A cavity-endfire, antenna system as, described i i claim 13 whereinsaid fully reflecting plate! includes aeree. flecting' 'rim of preselected width surrounding and con-Ll nected thereto. L

T D' IAIE PAIBN 2,644,091 1 6/1953. Middlemarlr-larz; ELI LIEBERMAN, Primary Baas... ,UQsQ'ct Y 343 -833, 837 j 

